For mobile phones, batteries with a higher capacity are desired, but it is quite difficult to increase the capacity of secondary batteries. Thus direct methanol fuel cells (DMFC) using methanol fuel become of greater interest.
DMFC has the advantage of possible size reduction since they can utilize liquid fuel directly without converting it into hydrogen or the like. Research efforts have been made thereon toward commercial use. However, the problems that the electrolyte membrane has a high methanol permeability and the anode catalyst has a low methanol oxidation activity arrest the commercial application of DMFC.
Most often PtRu catalysts are used as the anode catalyst. While efforts are made to search for high activity catalysts other than PtRu, no other catalysts have overtaken PtRu. Means for enhancing the activity of PtRu catalysts include use of a supported catalyst in which PtRu nano-particles having a small particle size and a large surface area are dispersed on a carbon support. Notably, commercially available supported catalysts, for example, TEC61E54 (54 wt % PtRu/C, Tanaka Kikinzoku Group, PtRu size 4 nm) still have an insufficient activity, with a further enhancement of activity being needed. To this end, it is desired that PtRu particles be further reduced in particle size (less than 4 nm) and more richly and uniformly supported on a carbon support (high loading and high dispersion).
Catalysts are prepared, for example, by an immersion technique. This technique involves admitting support carbon into an aqueous solution of a platinum or other metal precursor, impregnating the carbon with the metal precursor, separating the impregnated carbon, and chemically reducing with hydrogen or the like, whereby the reduced metal is loaded on the support carbon. Y. Takasu et al., Journal of Electrochemical Society, 147 (12), 4421-4427, 2000 (Non-Patent Document 1) describes high-dispersion PtRu-laden carbon prepared by the immersion technique, which has a metal loading of 30 wt %. A reducing temperature of 200 to 450° C. is necessary, and PtRu particles have an average particle size of 3 to 4 nm. An attempt to further increase the metal loading by the immersion technique results in PtRu particles having a larger average particle size and hence a smaller active surface area. That is, the immersion technique is difficult to achieve a high loading of fine size PtRu particles in a good dispersion.
Another catalyst preparation method is a colloidal method. In this method, PtRu colloids are prepared in solution and then loaded on support carbon. This method has advantageous abilities to control the size of PtRu particles and reduce the particle size distribution. Preparation of PtRu-laden catalyst by the Bonnemann method is described in T. J. Schmidt et al., Journal of Electrochemical Society, 145 (3), 925-931, 1998 (Non-Patent Document 2). Using PtCl2 and RuCl3 as reactants in THF solvent, a monohydride as a reducing agent, and tetraoctyl ammonium as a protective agent in a dry Ar atmosphere, colloids of 1.7±0.5 nm are prepared. In this document, however, the loading on support carbon is as low as 20 wt %. Further, since heat treatment at high temperature is necessary to remove the protective agent, the heat treatment causes agglomeration of PtRu particles to reduce the specific surface area of PtRu, which is problematic even if a high loading of PtRu is possible. Namely, even if PtRu colloids having a small average particle size are prepared, it is difficult to accomplish higher loading and higher dispersion while maintaining the particle size.                Non-Patent Document 1: Y. Takasu et al., Journal of Electrochemical Society, 147 (12), 4421-4427, 2000        Non-Patent Document 2: T. J. Schmidt et al., Journal of Electrochemical Society, 145 (3), 925-931, 1998        